1. Field of the Invention
The present invention relates to a mobile telephone system, and more particularly to a mobile telephone system which adopts the site diversity reception wherein a signal from a mobile station is received by a plurality of base stations to choose the most available base station.
2. Description of the Related Arts
Mobile telephone systems such as portable telephone systems have been popularized widely. In a mobile communication system of the cellular system, for example, each of a plurality of base stations has an area called cell covered by it, and the cells are disposed without any gap among them to allow a mobile station to communicate over a wide area.
In the cellular mobile telephone system, when a mobile station moves into a new cell, the communication is switched over from the present base station to the base station which covers the new cite without being suffered from interruption of the communication. The switch over of communication is called as xe2x80x9chand overxe2x80x9d.
Where a plurality of cells form an area in which they overlap each other on the boundary between or among them, if a mobile station roams into the overlapping area, it can communicate with all of the base stations which cover the overlapping area, or in other words, can communicate with a plurality of base stations simultaneously.
This fact is utilized in a conventional mobile telephone system (for example, a spread spectrum system) so that, when a mobile station is roaming around the boundary of two base stations, the mobile station communicates with the two base stations simultaneously. The method just described is adopted by the diversity hand over which is disclosed, for example, in xe2x80x9cFrom the Beginning to Applications of the CDMA (Code Division Multiple Access) Techniquexe2x80x9d by Takuro Satoh, published by Linearize Company.
The diversity hand over described in the xe2x80x9cCDMA Techniquexe2x80x9d is one of the communication system described above, a reception of an upward link signal from a mobile station to base station is utilized a site diversity reception among a plurality of related base stations.
The conventional site diversity reception among related base stations selection is described below.
FIG. 1 is a schematic block diagram of a usual mobile telephone system.
Referring to FIG. 1, base station 1 covers cell 2, and base station 3 covers cell 4. Base station 1 and base station 3 communicate through base station controller 6. Connector 7 to different networks is, for instance, a mobile communication switch or a gateway switch for connecting to a public telephone network.
The conventional mobile telephone system has the space diversity configuration. In particular, when cell 2 and cell 4 have an overlapping area among them and mobile station 5 is roaming within the overlapping area as seen in FIG. 1, mobile station 5 communicates with both of base stations 1 and 3 simultaneously. The signals received by the two base stations from mobile station 5 are combined by a host station, i.e., base station controller 6, and then transmitted to the other party of communication.
FIG. 2 is a block diagram showing a structure of a conventional base station and base station controller.
Base station 1 includes receiver 1b for receiving a radio signal from mobile station 5 through reception antenna 1a, analog to digital converter 1c for converting the analog signal from receiver 1b into a digital signal, counter spreader 1d for counter spreading the signal from analog to digital converter 1c, decoder 1e for decoding the signal from counter spreader 1d, error detector 1f for detecting whether the signal from decoder 1e contains some error or not, and reception level measurement unit 1g for measuring the reception level of the signal from counter spreader 1d. The signal from mobile station 5 after the error detection by error detector 1f is transmitted to base station controller 6. Also the information of a result of the detection by error detector 1f representing whether some error is included or not is transmitted to base station controller 6. Further, also the reception level measured by reception level measurement unit 1g is transmitted to base station controller 6.
Also base station 3 similarly includes receiver 3b, analog to digital converter 3c, counter spreader 3d, decoder 3e, error detector 3f, and reception level measurement unit 3g. Base station 3 transmits a signal from mobile station 5 after error detection by error detector 3f to base station controller 6, transmits information of a result of detection by error detector 3f whether some error is found or not to base station controller 6, and further transmits a reception level measured by reception level measurement unit 3g to base station controller 6.
Base station controller 6 includes selective composition controller 6m, data buffer 6b for storing a signal from base station 1, and data buffer 6c for storing a signal from base station 3.
FIG. 3 is a block diagram showing an internal structure of selective composition controller 6m shown in FIG. 2.
A signal from mobile station 5 transmitted through base station 1 is inputted to and stored into data buffer 6b. A signal from mobile station 5 transmitted through base station 3 is inputted to and stored into data buffer 6c. 
Selective composition controller 6m includes no error data selector 6f, and highest reception level data selector 6k. 
No error data selector 6f refers to information of presence/absence of an error received from base stations 1 and 3 and controls data buffers 6b and 6c to select that one of the data stored in data buffers 6b and 6c (signals from mobile station 5 received through base stations 1 and 3) which includes no error.
If both information of presence/absence of error received from base stations 1 and 3 indicates absence of error, then either information may be selected. On the other hand, if both information of presence/absence of error received from base stations 1 and 3 indicates presence of error, no error data selector 6f informs highest reception level data selector 6k of the presence of error.
If both information of presence/absence of error received from base stations 1 and 3 indicates presence of error, then highest reception level data selector 6k refers to reception levels received from base stations 1 and 3, and controls data buffers 6b and 6c to be selected either one having higher reception level data.
In this manner, that one of the data stored in data buffers 6b and 6c which is selected by selective composition controller 6m is transmitted to connector 7 to a different network.
As described above, in the conventional mobile telephone system, a signal from a mobile station to a base station (upward link signal) is received by a plurality of base stations, and each of the base stations performs error detection detecting an error detection signal included periodically (for example, at intervals of 10 ms) in the received signal and measurement of the reception level. Then, each of the base stations sends results of the error detection and the measurement of the reception level to a base station controller which is connected to the base stations and acts as a controlling apparatus to them. The base station controller selects one of the signals sent thereto from the base stations which is free from an error, but if all of the received signals contain an error, then the base station controller selects that one of the received signals which has the highest reception level, thereby to assure augmented reception quality.
In a mobile telephone system, a signal is divided into units called frames before it is transmitted from a mobile station to a base station. Error detection code mentioned above is provided for each frame formed with a time width of, for example, 10 ms, so that a transmission error can be detected in units of a frame.
FIG. 4 is a diagrammatic view showing an example of a structure of a frame used in a mobile telephone system.
As seen in FIG. 4, the frame shown is formed with a time width of 10 ms and includes user information bits which are real information transmitted by a mobile station and CRC (Cyclic Redundancy Check) check bits which form error detection codes.
However, in a conventional selection diversity system, if all of signals transmitted from a base station contain an error, then a base station controller selects one of the signals which has the highest reception level. Therefore, a signal transmitted to a connector to a different network in this instance contains an error without exception.
Where a signal in the form of a voice signal is transmitted, conversation is possible even if the signal includes some error. However, advancement of multimedia technology in recent years has increased transmission of data of images and the like information. Where the signal to be transmitted is data of images or the like, even a small amount of data error disables the value of data. Therefore, a mobile telephone system which allows further reduction of the error rate than ever is demanded.
It is an object of the present invention to provide a mobile telephone system of the selective diversity type which can transmit data with a reduced error rate.
In order to attain the object described above, according to an aspect of the present invention, there is provided a communication system, wherein communication information from a transmitter is received by a plurality of receivers, and if all of the communication information received by the receivers contain a communication error, then communication information free from a communication error is produced based on the communication information received by one of the receivers and the communication information received by the other one or ones of the receivers.
According to another aspect of the present invention, there is provided a mobile telephone system which employs the among base stations selective diversity wherein a signal from a mobile station is received by a plurality of base stations and the base stations transmit the respective received signals to a base station controller, wherein the base station controller produces a signal free from a communication error based on the signal from a predetermined one of the base stations and the signal or signals from the other one or ones of the base stations.
The mobile telephone system according to claim 2 of the present invention is characterized in that each of the signals received through the base stations by the base station controller includes a frame composed of user information bits which are real information transmitted from the mobile station and CRC check bits which form error detection codes, and the base station controller divides each of frames from the base stations into units of a predetermined size and rearranges the units into several combinations to obtain a signal free from a communication error.
The mobile telephone system according to claim 2 of the present invention is characterized in that the base station controller compares the signals received through the base stations with each other in units of a bit and changes the value of a bit with regard to which the result of the comparison does not exhibit coincidence to produce a signal free from a communication error.
According to a further aspect of the present invention, there is provided a mobile telephone system which employs the among base stations selective diversity wherein a signal from a mobile station is received by a plurality of base stations and the base stations transmit the respective received signals to a base station controller, wherein the signal received from the mobile station by the base stations is a signal of Reed-Solomon codes, and the base station controller compares the signals received through the base stations with each other in units of a bit, estimates that a symbol which contains a bit with regard to which the result of the comparison does not exhibit coincidence is an erroneous symbol, and uses the result of the estimation as error symbol position information in Reed-Solomon decoding to augment the error correction capability by the Reed-Solomon decoding.
The mobile telephone system according to claim 3 of the present invention is characterized in that the signal received from the mobile station by the base stations is a signal of convolutional codes and the base station obtains reliability information which represents a magnitude of reception energy for each of units divided from the frame when each of the base stations Viterbi decodes the signal, and when the base station controller changes a combination of the units, the base station controller selects that one of the units with regard to which the reliability information exhibits the highest reliability as a rearrangement candidate thereby to decrease the processing time required to find out a no error frame.
The first characteristic of the present invention resides in that, as described in connection with the first embodiment, when all of the signals received by the receivers in the among base stations selective diversity contain a communication error, the opportunity to obtain a no error signal can be increased by dividing the signal received by each of the base stations into a plurality of units and performing reconstruction and error re-detection for all possible combinations of the divided units of the received signals.
The second characteristic of the present invention resides in that, as described in connection with the second embodiment, when all of the signals received by the receivers in the among base stations selective diversity contain a communication error, the opportunity to obtain a no error signal can be increased by performing comparison of the signals received by the base stations with each other in units of a bit and performing reconstruction and error re-detection for all possible combinations of information bits at positions of the signals at which the signals are different from each other.
The third characteristic of the present invention resides in that, as described in connection with the third embodiment, when all of the signals received by the receivers in the among base stations selective diversity contain a communication error, the error correction capability by Reed-Solomon decoding can be augmented by comparing the signals received through the base stations with each other to detect a position at which the signals are different from each other and using the position as error symbol position information in Reed-Solomon (RS) decoding used for data communication.
The fourth characteristic of the present invention resides in that, although a frame reconstruction function and an error detection function are additionally provided as new functions to the base station controller, a conventional communication protocol can be used as the communication protocol between the base stations and the base station controller.
The fifth characteristic of the present invention resides in that, as described in connection with the fourth embodiment, when all of the signals received by the receivers in the among base stations selective diversity contain a communication error, by dividing each of the signals received by the base stations into a plurality of units, determining reliability information for each of the signal units and reconstructing a frame with the divided signal units selectively used in a descending order of the reliability, a combination of no error signal units can be find out rapidly.